A device of the kind described above is known through our own EP-A-94200571.1, to which reference is made for further information about the technical field and certain features common for this prior device and the device according to the present invention. With this known device it has become possible to obtain a reasonably even or non-fluctuating differential rotational speed decrease or braking action in the device. With one embodiment of the prior device it is also possible to obtain the desired, even action also in a second rotational direction, i e in the reverse gear, if the device is used as a so called differential brake in a connection between the front and the rear axle of a road vehicle.
However, a supply piston in the hydraulic arrangement of the prior device is spring biassed into engagement with its cam, and the hydraulic system itself is of the open type, sucking oil from a reservoir and delivering it back after use in the system.
A drawback with this device is that it is impossible to obviate or switch-off its action, which for example means that towing away of a vehicle provided with such a device is very difficult. A present requirement on a device of the kind referred to is that it shall make a towing away at a speed of 50 km/h possible.